1. Field of the Invention
The present invention relates to a fuel cell which uses as a fuel a reducing agent such as pure hydrogen or modified hydrogen obtained from methanol or fossil fuels and as an oxidizing agent air or oxygen, and more particularly to an assembly of a solid polymer electrolyte membrane and electrodes, and a method for the manufacture of the assembly.
2. Description of Prior Art
One of the most important factors which govern the discharge performance of solid polymer type fuel cells is the reaction area at an interface of three phases formed by pores which are passages for feeding of reaction gas, a solid polymer electrolyte having protonic conductivity due to containing of water, and an electrode material of electronic conductor at the interface between a solid polymer electrolyte membrane and an electrode.
Hitherto, in order to increase the three phase interface, it has been attempted to apply a layer prepared by mixing and dispersing an electrode material and a solid polymer electrolyte to the interface between the membrane and a porous electrode. For example, Japanese Patent Kokoku (Examined Publn.) Nos. 62-61118 and 62-61119 disclose a method which comprises coating a mixture of a solution of solid polymer electrolyte with a catalyst compound on a solid polymer membrane, hot pressing the coated membrane on an electrode material and then reducing the catalyst compound or carrying out the coating after the reduction and then carrying out the hot pressing.
Japanese Patent Kokoku (Examined Publn.) No. 2-48632 employs a method which comprises molding a porous electrode, sprinkling a solution of an ion-exchange membrane resin on the electrode and hot pressing the electrode and the ion-exchange membrane. Furthermore, Japanese Patent Kokai (Unexamined Publn.) No. 3-184266 uses a powder prepared by coating a solid polymer electrolyte on the surface of a polymer resin, Japanese Patent Kokai (Unexamined Publn.) No. 3-295172 employs a method which comprises incorporating a powder of a solid polymer electrolyte into an electrode. Japanese Patent Kokai (Unexamined Publn.) No. 5-36418 discloses a method which comprises mixing a solid polymer electrolyte, a catalyst, a carbon powder and a fluoropolymer and forming the mixture into a film to form an electrode.
All of the above patent publications use solvents of alcohols for the solutions of the solid polymer electrolyte. Furthermore, U.S. Pat. No. 5,211,984 reports a method which comprises preparing an inky dispersion comprising a solid polymer electrolyte, a catalyst and a carbon powder using glycerin or tetrabutylammonium salt as a solvent, casting the dispersion on a polytetrafluoroethylene (hereinafter referred to as "PTFE"), and then transferring it onto the surface of a solid polymer electrolyte membrane or a method which comprises changing the exchanging group of a solid polymer electrolyte membrane to Na type, coating the above inky dispersion on the surface of the membrane and heating and drying the coat at 125.degree. C. or higher to again change the group to H type.
In order to realize the high output density which is a feature of solid polymer type fuel cells, it is important to form feeding channels for reaction gas (gas channel) in the electrode catalyst layer to enhance the performance to feed the gas to the reaction site. Therefore, it has been attempted to add a water repellent material such as a fluoropolymer and to form a gas channel in the layer.
For example, in Japanese Patent Kokai (Unexamined Publn.) No. 5-36418, PTFE powders and carbon powders supporting a catalyst are dispersed in a solution of a solid polymer electrolyte and kneaded and a catalyst layer is formed therefrom. Furthermore, in Japanese Patent Kokai (Unexamined Publn.) No. 4-264367, an electrode is prepared using a mixed solution of carbon powders supporting a catalyst with a colloid solution of PTFE.
Furthermore, J. Electroanal. Chem. 197 (1986) describes on page 195 that carbon powders subjected to water repelling treatment with PTFE are mixed with carbon powders supporting a catalyst and a gas-diffusible electrode for acidic electrolyte is prepared therefrom. In U.S. Pat. No. 5,211,984, a catalyst layer of electrode is prepared using only a solid polymer electrolyte, a catalyst and a carbon powder without using the water repellent material mentioned above.
However, the conventional methods disclosed in the above patent publications suffer from the problem that no sufficient reaction area can be ensured at the interface between the electrode and the ion-exchange membrane because of insufficient degree of contact between the solid polymer electrolyte and the catalyst.
Furthermore, when the dispersion with alcoholic solvents is coated on a porous substrate or when the inky dispersion is coated on a porous substrate, the dispersion cannot be directly molded on the surface of the substrate as the dispersion penetrates or permeates into the inside of the substrate and thus, complicated processing techniques such as transferring are needed.
Moreover, the above-mentioned method of directly coating the inky dispersion on the surface of the membrane requires the complicated production technique of replacing the exchange group of the membrane many times.
The method of adding a fluoropolymer has the defect that the catalyst particles are coated excessively with the fluoropolymer and the reaction area diminishes to cause deterioration of polarization characteristics. On the other hand, if the carbon powder subjected to water repelling treatment with PTFE is used as described in J. Electroanal. Chem., coating of the catalyst particles with PTFE can be controlled, but no investigation has been made on the effects of addition of the water repelled carbon powder or amount of the carbon powder added in case the solid polymer electrolyte is used. Further, when the electrode is made of only the catalyst-supporting carbon powder and the solid polymer electrolyte, there are problems that the cell voltage at a high current density decreases or becomes unstable due to flooding of water produced.